feat(auth): add SignupForm component with validation and password toggle

Form with name/email/password fields, client-side validation,
inline error messages, and password show/hide toggle.
Uses native form action for progressive enhancement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package-lock.json

@@ -19,6 +19,7 @@
 				"@tailwindcss/vite": "^4.2.2",
 				"@testing-library/jest-dom": "^6.9.1",
 				"@testing-library/svelte": "^5.3.1",
+				"@testing-library/user-event": "^14.6.1",
 				"@types/node": "^25.5.0",
 				"@vitest/ui": "^4.1.2",
 				"jsdom": "^29.0.1",
@@ -1809,6 +1810,20 @@
 				"svelte": "^3 || ^4 || ^5 || ^5.0.0-next.0"
 			}
 		},
+		"node_modules/@testing-library/user-event": {
+			"version": "14.6.1",
+			"resolved": "https://registry.npmjs.org/@testing-library/user-event/-/user-event-14.6.1.tgz",
+			"integrity": "sha512-vq7fv0rnt+QTXgPxr5Hjc210p6YKq2kmdziLgnsZGgLJ9e6VAShx1pACLuRjd/AS/sr7phAR58OIIpf0LlmQNw==",
+			"dev": true,
+			"license": "MIT",
+			"engines": {
+				"node": ">=12",
+				"npm": ">=6"
+			},
+			"peerDependencies": {
+				"@testing-library/dom": ">=7.21.4"
+			}
+		},
 		"node_modules/@types/aria-query": {
 			"version": "5.0.4",
 			"resolved": "https://registry.npmjs.org/@types/aria-query/-/aria-query-5.0.4.tgz",